This invention relates to a high voltage power distribution and collection system and more particularly to a high voltage converter system for distributing and collecting power to and from a set of isolated, geographically-scattered, or generally inaccessible loads and sources, interconnected by one or more high voltage power lines or cables.
In typical signal-carrying cable installations, repeater stations are required at various intervals, typically ten or more miles apart, to restore and amplify attenuated and time-smeared signals. These repeater stations are electronic devices which require electric power to operate.
Operating power for repeater stations is typically supplied by conventional power supplies connected to a local power grid at each repeater station, but, if grid power is not available or the repeater station is not readily accessible, such as repeater stations distributed at various intervals along the length of an intercontinental undersea cable, power for these repeater stations, or any other power consuming electronic devices, must be supplied at the cable ends and transmitted through the cable itself to the repeater stations, typically by means of a high voltage conductor (high voltage line) incorporated in the cable.
In the generation, transmission, and distribution of conventional electrical power, power is transmitted over long distances at high AC voltages, with the voltage step-down for local use or distribution done by AC transformers. High DC transmission voltages are also used, with down-conversion to AC being done by elaborate regional terminal installations. Conventional, long-distance, AC transmission techniques can not be used with undersea cables and similar applications for a variety of reasons, such as the capacitance effects and interference with signals. In applications like the powering of undersea repeaters, the power is most efficiently transmitted as low-current, high voltage DC introduced at the accessible ends of the cable, even though this creates a difficult voltage down-conversion or limitation problem at each of the remote locations which use the transmitted power. For ordinary electronic circuitry, the transmitted power must be converted (or limited) to low voltage at each individual locality or site where it is to be used.
Unfortunately, the existing and historical methods used for down-converting high voltage DC power to low DC or AC voltage are not at all appropriate for low or moderate power applications, since these methods typically involve the use of large SCR or mercury vapor rectifier tube installations requiring constant attention, or (for very low power installations) the use of power-wasting dropping resistors. A self-contained, reliable, high voltage DC-to-low voltage DC (or HVDC to AC) power conversion device able to run unattended is needed.
Likewise where power is to be collected from inaccessible, geographically-scattered, low-voltage sources of power (such as windmills, tidal generators, solar cells, and the like), an AC- or DC-to-HVDC converter capable of stepping up the low voltage power to high voltage before placing it on the high voltage cable is necessary, and this converter must be able to run unattended.
Furthermore, in a situation where any given device might act as either a source or a sink of power depending on local load or generating conditions, a converter readily capable of operation as either a voltage-converting step-down device or xe2x80x9cin reversexe2x80x9d as a voltage step-up device in response to the direction of power flow would be very desirable.
At modest high voltages (less than 1000 VDC) and at the few-watt to several-kilowatt power levels, the power conversion or power conditioning functions are readily accomplished by conventional switching-type power supplies commonly used to convert high voltage DC to low voltage DC (or high voltage DC to low voltage AC). These prior art techniques are, however, unavailable for high voltage service above 1000-3000 volts due to the voltage limitations of the opening and closing semiconductor switches (IGBTs) used as choppers or synchronous rectifiers. Obtaining the required voltage holdoff for high voltage operation by connecting two or more switches of lesser holdoff capability in series and then switching them on and off simultaneously is notoriously difficult to do, because it requires high stability in voltage division across the switching devices, and great accuracy in the timing of their operation. If the voltage divides unevenly, or if one switch lags even slightly, to where the entire voltage appears across that switch, damage to the switch occurs instantly. Once damaged, it does not recover. For this reason, series-connected stacks of semiconductor switch devices are notoriously subject to so-called xe2x80x9czipperxe2x80x9d failure, wherein the collapse of one device can set off a progressive and catastrophic destruction of all the others in the stack.
Other prior art switches have similar limitations. Sustained, repetitive opening and closing cannot be done efficiently with high voltage closing switches, such as thyratrons, spark gaps, or SCRs, since these devices, once closed, lose control and have to be shut off by other means. Some of these prior art devices also have negative current-voltage characteristics, uncertain triggering delays, and unstable holdoff recovery characteristics, making a non-wasteful, controlled, and reliable switch operation with them extremely difficult to accomplish. Repetitive high voltage switching by vacuum tubes (hard tubes) can also be done, but the technique is wasteful and inefficient due to the low current-emitting capability of the cathodes, the filament power required, the power lost in anode dissipation, and the size, fragility, and relatively short life expectancy of the tubes. In short, in the prior art, it has not been possible to switch high voltage reliably enough above 1000-3000 volts DC to make suitable high voltage switching supplies for converting high DC voltages (3000-100,000 volts or more) to low voltage DC or AC.
In the absence of suitable supplies able to be connected independently and directly to the high voltage line, other means have been used to supply power to devices to be operated in geographically isolated and inaccessible locations. One typical prior art technique of providing power to operate remotely located repeater stations or electronic devices is to connect them in series along the cable, and then force the maximum DC current needed by any given remote device through the entire system. This is typically accomplished by connecting a high voltage DC supply of one polarity at one end of the cable and another high voltage supply of opposite polarity at the other end of the cable to establish a current flow, with current return taking place by conduction through the earth. These cable-feeding, main DC power supplies are connected this way to obtain the required current at only half the voltage stress on the cable insulation with respect to ground that would otherwise occur if power were being fed from only one end. The current is passed through all the repeater stations located on the cable. The result is a series configuration of repeater stations on the high voltage power line with the necessary operating voltage at each remote location being developed by the voltage drop obtained across the equivalent input resistance of each successive load (e.g., series-connected street-lights or Christmas tree lights are powered in exactly the same way).
While the series connection assures that for a given wire size and power supply voltage, all devices will have the same current available (since current in a series circuit is the same everywhere), this prior art technique causes all the voltage drops due to cable and the power supplies of each repeater station to add along the cable length. Where a number of separate repeaters are to be powered, and the intervening cable resistance voltage drops overcome, very high voltages may be required to force the maximum desired operating current through the entire series system. As a result, cable repeater systems are typically fed with 10,000-volt main DC power supplies of opposite polarity at either end of the cable, the high voltage being necessary to develop the necessary current, typically of the order of one ampere, through the resistance of the cable and the loads.
A consequence of this prior art design is that the main supply voltage starts out high at one polarity at the near end of the cable, falls through zero somewhere in the middle, and then continues to fall with respect to the near end (i.e. it then rises toward the full voltage of the opposite polarity power supply at the far end). Since each low voltage output is supplied relative to the main supply voltage at its location, each repeater station (with the possible exception of a repeater station that might happen to be at the exact electrical center of the series system) operates at an elevated positive or negative voltage with respect to ground. Therefore, each repeater station power supply must be isolated from ground with careful insulation, and any equipment connected to it must be similarly isolated, meaning that the insulation cannot be breached for repair or reconfigured where such a breach would bring the equipment inside in contact with grounded tools or (necessarily-grounded) sea-water. Working on such equipment while it is running would be dangerous for obvious reasons: such work would have to be done xe2x80x9chotxe2x80x9d with the worker and his tools electrically isolated from ground or from contact with sea water. It also follows that accidents, opens, or faults to ground at any given location would disable the entire system, again much as faults or opens will disable entire system, similar to strings of streetlights or Christmas-tree bulbs.
This prior art method for supplying power to remotely located repeater stations suffers from another disadvantage. If the amount of current needed at any one location is less than the line current (current which is perforce sized to accommodate the current drawn by the maximum load), then wasteful compensating resistors must be used to re-route the unneeded current at that location around the under-consuming load while still maintaining the correct voltage across it. Likewise, local compensating resistor adjustments must be made to counter the effects of load changes taking place elsewhere in the system. At constant line current the available power can be adjusted by changing the total system voltage drop, but this method of control requires careful attention to stability.
Another set of disadvantages arise from the typical behavior of electrical loads in series. For stability, the repeater stations must present a constant current load, or one that shares a common resistance-current characteristic. A repeater station that happened to have a more strongly positive resistance-current characteristic than the other repeater stations, for example, from a repeater station having a shorter time-constant for heating due to operating dissipations, that repeater station will monopolize power at that location, leading to possible thermal runaway and burnout. A repeater station with an incompatible negative resistance-current characteristic will produce local power starvation and may cause undesirable oscillations in the current drawn by the entire system.
Another disadvantage found in the prior art is that while collection of power from distributed low voltage power generating sources (or from a mixture of low voltage loads and sources), could in principle be done with all the varying sources and loads connected in series, the result would be an impractical, hard-to-control system subject to large voltage fluctuations and current oscillations.
It is therefore an object of this invention to provide an improved system for distributing and collecting power, especially suited for use with multiple, isolated, inaccessible loads or sources connected to a high voltage DC transmission line or cable, where individual DC-to-DC (or DC-to-AC) power converters are connected directly across the across high voltage supply lines, or between a high voltage line and ground.
It is a further object of this invention to provide such a high voltage converter system which efficiently provides regulated and isolated low voltage power to a plurality of remotely located repeater stations or other electronic devices on a high voltage power line.
It is a further object of this invention to provide such a high voltage converter system which eliminates the need to configure multiple repeater stations in series along the high voltage power line.
It is a further object of this invention to provide such a high voltage converter system which eliminates the need for power-wasting compensating circuits or devices.
It is a further object of this invention to provide such a high voltage converter system which reduces the power required to provide regulated and isolated power to multiple remotely located stations or electronic devices.
It is a further object of this invention to provide such a high voltage converter system which reduces the amount of insulation and isolation needed for the low voltage output side of power supplies used to power remotely located repeater stations or electronic devices on a high voltage power line, and for any equipment attached to these low voltage outputs.
It is a further object of this invention to provide such a high voltage converter system which can tolerate breaching of the low voltage containment or insulation provided for repeater station equipment in an undersea environment.
It is a further object of this invention to provide such a high voltage converter system in which remotely located repeater stations can be easily accessed and repaired without requiring elaborate precautions for personnel and equipment protection occasioned by the need to work on equipment xe2x80x9cfloatingxe2x80x9d at high voltage.
It is a further object of this invention to provide such a high voltage converter system in which remotely located repeater stations can be easily accessed and repaired, or can sustain damage, or can be removed from service without interrupting the power- and signal-carrying functions of the cable.
It is a further object of this invention to provide such a high voltage converter system which can regulate itself in response to local demands from the cable repeater, without requiring readjustment of all the other loads or load compensating circuits in a cable string.
It is a further object of this invention to provide such a high voltage converter system which eliminates a possible cause of thermal runaway and burnout in remotely located repeater stations.
This invention results from the realization that a truly robust high voltage converter system which supplies isolated and regulated low voltage power to remotely located repeater stations on a high voltage power line, such as an undersea cable, can be achieved, not by connecting the power supplies of repeater stations in series along the length of the high voltage power line, a scheme which wastes power and requires each repeater station to be carefully insulated and isolated, or by using dropping resistors at each location, which waste even more power, but instead by utilizing converters which feature a plurality of controllable high voltage opening and closing switches connected in series which function effectively and efficiently at high voltages above 1000 volts to draw power directly from (or replace on) the high voltage line at each remote or inaccessible location by periodically chopping or reversing the voltage applied to the primary winding of a step-down transformer; if power is being fed back onto the high voltage line, the chopping or voltage-reversing is done to the power applied to the transformer""s low voltage winding, which is now used as a primary, and the power taken from the high voltage winding, now used as a secondary, is synchronously rectified by the series string (or strings) of high voltage switches, to yield high voltage DC power which can be placed back on the high voltage line.
This invention features a high voltage converter system comprising a high voltage power line at a first voltage level, a step-down device connected to a second voltage level lower than the first voltage level, a high voltage switching module including a plurality of controllable switch elements in which the voltage across each the controllable switch element is limited to a predetermined voltage when each the voltage limiting controllable switch element is in an open state and to a zero voltage when each the voltage limiting controllable switch element is in a closed state. In one example, the voltage limiting controllable switch elements are typically connected in series between the power line and the step-down device. The high voltage converter system also includes a control system to selectively set the open state and the closed state of each the voltage limiting controllable switch elements to control the current flowing through the step-down device in response to the potential difference between the first and second voltage levels.
In one embodiment, each the controllable switch element includes a controllable switch capable of both opening and closing, and a voltage limiting device connected in parallel with each controllable switch. In one example, the control system includes a control circuit configured to activate at least one gate drive to drive an isolation circuit to control a sequence of open and closed states of the controllable switch elements to control the flow of current through the step-down device. Typically, the voltage applied to the step down device is in the range of 1000 to 100,000 volts or more, depending on the number of switching elements connected in series.
In one example, there are at least two gate drives, at least two gate drive transformers, and at least two controllable switch elements. Ideally, the step-down device is a high voltage step-down transformer. In one example, the high voltage step-down transformer is connected to a conditioning circuit to provide low voltage DC output. The high voltage step-down transformer may be connected to a conditioning circuit to provide low voltage AC output. The conditioning circuit may include a rectifier and a filter. Typically, the low voltage AC or DC output is in the range of 0 to 1000 volts. In one design, the high voltage converter may include a repeater power supply station connected to the conditioning circuit. In other examples, a motor or other electrical or electronic load may be connected to the conditioning circuit.
In another embodiment of this invention, the high voltage converter system includes an auxiliary low voltage power supply which receives reduced voltage from the high voltage power line for providing initial and operating power to the control circuit, for charging a battery, and for providing power to a gate drive power supply. A resistance may be connected between the high voltage power line and the auxiliary low voltage power supply to reduce the voltage received by the auxiliary low voltage power supply. Ideally, the battery is charged by the low voltage DC output provided by the conditioning circuit. The system may also include means for detecting modulated control signals on the high voltage power line and means for demodulating the modulated signals to activate the control circuit to define the voltage applied to the step-down device. Typically, the first voltage level in the range of 1000 to 100,000 volts and the second voltage level is in the range of 0 to 1,000 volts, but may be any voltage needed by the connected equipment. In one preferred embodiment, the high voltage power line is a power-carrying conductor in an undersea cable. In one preferred design, the step-down device, the high voltage modulator, and the control system are oil immersed or epoxy encapsulated for isolation, cooling, pressure resistance.
In other designs, each the voltage limiting controllable switch element limits the voltage across each the voltage limiting controllable element circuit to a predetermined voltage when each the voltage limiting controllable switch circuit is in an open state and to zero voltage when each the voltage limiting controllable switch element is in a closed state.
In one embodiment the high voltage switch module is connected in a in reverse configuration to accept low voltage AC or DC power from an isolated, inaccessible sources, the module chopping or conditioning the low voltage AC or DC power as necessary to transform the power to high voltage AC power, the module further synchronously rectifying the power by utilizing of one or more sets of the voltage-limited switching and returning the high voltage AC power to the high voltage line. In one embodiment, the high voltage switch module is configured to automatically and dynamically responding to the direction of power flow on the high voltage power line by behaving as a step-down converter when the power flow is in a first direction and as a step-up converter when the power is flowing in a second direction opposite the first direction, the switch module thereby responding to changing local loads and changing outputs of power from local sources
This invention further features a high voltage down-converter system comprising a plurality of stations interconnected between a high voltage power line at a first voltage level and a common voltage lower than the first voltage level. Each the station typically includes a step-down device connected to the common voltage, a high voltage switch module including a plurality of controllable switch elements in which voltage is limited across each the controllable switch element to a predetermined voltage when each the voltage limiting controllable switch circuit is in an open state and to a zero voltage when each the voltage limiting controllable switch element is in a closed state, the voltage limited controllable switch elements being connected in series between the power line and the step-down device, and a control system to selectively set the open state and the closed state of the voltage limiting controllable switch elements to control the passage of current through the step-down device.
In one embodiment, the common voltage may be ground. In one example, the common voltage is a second voltage power line at a voltage lower than the first voltage level, the second power line interconnecting each the station.
This invention further features a high voltage converter system for undersea cable repeater stations comprising a plurality of stations interconnected between a high voltage undersea power line at a first high voltage level and a common ground. Each the station ideally includes a step-down device connected to the common ground, a high voltage switching module including a plurality of controllable switch elements in which the voltage across each the controllable switch element is limited to a predetermined voltage when each the voltage limiting controllable switch element is in an open state and to a zero voltage when each the voltage limiting controllable switch element is in a closed state. In one example, the voltage limiting controllable switch elements may be connected in series between the power line and the step-down device. The high voltage converter system for undersea cable repeater stations further includes a control system to selectively set the open state and the closed state of each the voltage limiting controllable switch elements to control the current flowing through the step-down device in response to the potential difference between the first and second voltage levels, a conditioning circuit connected to the step-down device configured to output low voltage DC, and a repeater station connected to the conditioning device.
Ideally, the plurality of stations are oil immersed or epoxy encapsulated for isolation, cooling, and pressure resistance from undersea environment. In one embodiment, the high voltage undersea power line is at a voltage in the range of 1,000 to 100,000 or more volts. In one design, the low voltage DC is in the range of 0 to 1,000 volts or any voltage needed by the connected equipment.